While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, Vol. 30, pp. 322-334, 1969; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often >100V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g. <1.0 μm) between the anode and the cathode. Herein, the term “organic EL element” encompasses the layers between the anode and cathode electrodes. Reducing the thickness lowered the resistance of the organic layer and has enabled devices that operate at much lower voltage. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, therefore, it is referred to as the hole-transporting layer, and the other organic layer is specifically chosen to transport electrons, referred to as the electron-transporting layer. Recombination of the injected holes and electrons within the organic EL element results in efficient electroluminescence.
There have also been proposed three-layer organic EL devices that contain an organic light-emitting layer (LEL) between the hole-transporting layer and electron-transporting layer, such as that disclosed by Tang et al [J. Applied Physics, Vol. 65, Pages 3610-3616, 1989]. The light-emitting layer commonly consists of a host material doped with a guest material. Still further, there has been proposed in U.S. Pat. No. 4,769,292 a four-layer EL element comprising a hole-injecting layer (HIL), a hole-transporting layer (HTL), a light-emitting layer (LEL) and an electron transport/injection layer (ETL). These structures have resulted in improved device efficiency.
Many emitting materials that have been described as useful in an OLED device emit light from their excited singlet state by fluorescence. The excited singlet state is created when excitons formed in an OLED device transfer their energy to the excited state of the dopant. However, it is generally believed that only 25% of the excitons created in an EL device are singlet excitons. The remaining excitons are triplet, which cannot readily transfer their energy to the singlet excited state of a dopant. This results in a large loss in efficiency since 75% of the excitons are not used in the light emission process.
Triplet excitons can transfer their energy to a dopant if it has a triplet excited state that is low enough in energy. If the triplet state of the dopant is emissive it can produce light by phosphorescence. In many cases singlet excitons can also transfer their energy to lowest singlet excited state of the same dopant. The singlet excited state can often relax, by an intersystem crossing process, to the emissive triplet excited state. Thus, it is possible, by the proper choice of host and dopant, to collect energy from both the singlet and triplet excitons created in an OLED device and to produce a very efficient phosphorescent emission.
One class of useful phosphorescent materials are cyclometallated transition metal complexes having a triplet excited state. For example, fac-tris(2-phenylpyridinato-N,C2′)iridium(III) (Ir(ppy)3) strongly emits green light from a triplet excited state owing to the large spin-orbit coupling of the heavy atom and to the lowest excited state which is a charge transfer state having a Laporte allowed (orbital symmetry) transition to the ground state (K. A. King, P. J. Spellane, and R. J. Watts, J. Am. Chem. Soc., 107, 1431 (1985), M. G. Colombo, T. C. Brunold, T. Reidener, H. U. Gudel, M. Fortsch, and H.-B. Burgi, Inorg. Chem., 33, 545 (1994). Small-molecule, vacuum-deposited OLEDs having high efficiency have also been demonstrated with Ir(ppy)3 as the phosphorescent material and 4,4′-N,N′-dicarbazole-biphenyl (CBP) as the host (M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, S. R. Forrest, Appl. Phys. Lett., 75, 4 (1999), T. Tsutsui, M.-J. Yang, M. Yahiro, K. Nakamura, T. Watanabe, T. Tsuji, Y. Fukuda, T. Wakimoto, S. Miyaguchi, Jpn. J. Appl. Phys., 38, L1502 (1999)).
Another class of phosphorescent materials include compounds having interactions between atoms having d10 electron configuration, such as Au2(dppm)Cl2 (dppm=bis(diphenylphosphino)methane) (Y. Ma et al, Appl. Phys. Lett., 74, 1361 (1998)). Still other examples of useful phosphorescent materials include coordination complexes of the trivalent lanthanides such as Tb3+ and Eu3+ (J. Kido et al, Appl. Phys. Lett., 65, 2124 (1994)). While these latter phosphorescent compounds do not necessarily have triplets as the lowest excited states, their optical transitions do involve a change in spin state of 1 and thereby can harvest the triplet excitons in OLED devices.
Although many phosphorescent cyclometallated Ir complexes have been described as useful in an EL device, Pt-based organometallic complexes have not been examined as extensively. Some Pt phosphorescent complexes include cyclometallated Pt(II) complexes such as cis-bis(2-phenylpyridinato-N,C2′)platinum(II), cis-bis(2-(2′-thienyl)pyridinato-N,C3′)platinum(II), cis-bis(2-(2′-thienyl)quinolinato-N,C5′)platinum(II), or (2-(4,6-diflourophenyl)pyridinato-NC2′)platinum(II)acetylacetonate. However, those complexes do not give high efficiency or stability. Further, some of those complexes are not stable toward sublimation (S. Lamansky et al., WO 00/57676) and thus not suitable for vacuum-deposition to form organic films. Pt(II) porphyrin complexes such as 2,3,7,8,12,13,17,18-octaethyl-21H, 23H-porphine platinum(II) are reported in U.S. Pat. No. 6,048,630 as useful phosphorescent materials in an electroluminescent device although they did not give a very high luminance yield. Recently, C. Che, W. Lu, and M. Chan reported organometallic light-emitting materials based on (C^N^N) tridentate cyclometalated Pt(II) arylacetylides. (US 2002/0179885 and references cited therein). We have also disclosed organometallic light-emitting materials based on (C^N^N) tridentate cyclometallated Pt(II) complexes (U.S. Pat. No. 6,824,895 B1) and (N^C^N) tridentate cyclometallated Pt(II) complexes that have been disclosed in prior filing Ser. No. 10/729,238 filed on Dec. 15, 2003. Furthermore, the coordinatively unsaturated square planar Pt(II) complexes tend to aggregate at high concentration, leading to concentration quenching or the formation of eximers that can affect the hue of OLED devices made of them.
Further, Wei Lu et al reported tridentate cyclometallated Pt(II) complexes based on 2,6-diphenylpyridine (Wei Lu, Michael C. W. Chan, Kung-Kai Cheung, and Chi-Ming Che, Organometallics 2001, 20, 2477-2486). Although the complexes are photoluminescent in the solid state and in 77 K methanol/ethanol glass, no emission is detected in dichloromethane at room temperature. Therefore, the complexes do not appear to be highly efficient emitters useful in electroluminescent devices.
It is a problem to be solved to provide new organometallic materials that will function as phosphorescent materials in an electroluminescent device having improved luminance and luminescence efficiency.